Sexual orientation group disparities in several critical health domains are now well documented, with sexual minorities reporting more psychological distress, substance use and dependence, and eating disorders symptoms. Little is known about the reasons underlying higher risk behaviors among sexual minorities relative to heterosexuals. Herein, we propose to explain these sexual orientation disparities by testing our Sexual Minority Stress Theory (SMST). SMST is based on society's stigmatization of homosexuality. Gay-related stress, the experience by a sexual-minority individual of society's stigmatization, consists of external stressors (such as prejudice, discrimination, verbal abuse, physical violence) perpetrated by known individuals (such as kin, heterosexual friends) and strangers in the community. Gay-related stress also consists of internal stress, in which the sexual-minority individual internalizes society's stigmatization, such as possessing negative attitudes toward homosexuality. Our proposed research will address the impact of gay-related stress on disparities between and among sexual orientation subgroups in psychological distress, physiological distress (cortisol, DHEA), health behaviors, chronic pain, adaptation, and adjustment in emerging adulthood, and examine moderators, such as the effect of parent-child relationships on the relations between sexual orientation and gay-related stress and the effect of coping on the relations between gay-related stress and both psychological and physiological distress. Gay-related stress may be particularly important in influencing a sexual minority youth's development and psychosocial adjustment in emerging adulthood. Our research will be sensitive to important precursors of gay-related stress, such as gender nonconformity. It will also examine how sex, age at experiencing gay-related stress, sexual minority status and transitions in status, concordant vs. nonconcordant orientation patterns among sexual minorities, and sexual identity development vary with gay-related stress, health, adaptation, and adjustment outcomes. We will collect new self-report survey and biological data to investigate these critical issues in the Growing Up Today Study (GUTS), a longitudinal cohort study of 16,882 U.S. adolescents and emerging adults begun in 1996 and ongoing. Specifically, we will add 3 data collection phases to more than 10 years of data collected from GUTS and link the youths' data with those of their mothers, who comprise the Nurses' Health Study II. We will examine disparities in violence victimization in childhood and adolescence and other forms of gay-related stress and the impact of these over time on subsequent psychological and physiological distress, substance use and eating disordered behaviors, chronic pain, adaptation, and adjustment in emerging adulthood.